Methods of manufacturing semiconductor devices

ABSTRACT

Methods of forming semiconductor devices are disclosed. In some embodiments, a first trench and a second trench are formed in a substrate, and dopants of a first conductivity type are implanted along sidewalls and a bottom of the first trench and the second trench. The first and second trenches are filled with an insulating material, and a gate dielectric and a gate electrode over the substrate, the gate dielectric and the gate electrode extending over the first trench and the second trench. Source/drain regions are formed in the substrate on opposing sides of the gate dielectric and the gate electrode.

This application is a continuation application of co-pending U.S. patentapplication Ser. No. 14/641,067, filed on Mar. 6, 2015, entitled,“Method of Manufacturing Semiconductor Devices,” which is a continuationapplication of U.S. patent application Ser. No. 13/250,856, now U.S.Pat. No. 8,994,082, filed on Sep. 30, 2011, entitled “Transistors,Methods of Manufacturing Thereof, and Image Sensor Circuits with ReducedRTS Noise,” which application is hereby incorporated herein by reference

BACKGROUND

Semiconductor devices are used in a variety of electronic applications,such as personal computers, cell phones, digital cameras, and otherelectronic equipment, as examples. The semiconductor industry continuesto improve the integration density of various electronic components(e.g., transistors, diodes, resistors, capacitors, etc.) by continualreductions in minimum feature size, which allow more components to beintegrated into a given area.

A transistor is an element that is utilized extensively in semiconductordevices. There may be millions of transistors on a single integratedcircuit (IC), for example. A common type of transistor used insemiconductor device fabrication is a metal oxide semiconductor fieldeffect transistor (MOSFET). Early MOSFET processes used one type ofdoping to create either positive or negative channel transistors. Morerecent designs, referred to as complimentary MOS (CMOS) devices, useboth positive and negative channel devices in complementaryconfigurations. While this requires more manufacturing steps and moretransistors, CMOS devices are advantageous because they utilize lesspower, and the devices may be made smaller and faster.

One type of device that can be manufactured using CMOS processes is aCMOS image sensor (CIS). One problem facing CMOS image sensors is a highamount of random telegraph signal (RTS) noise in pixel source followertransistors, which decreases the image sensor sensitivity. RTS noise isoften reduced by including enlarged source follower transistors in CMOSimage sensors; however, including such large devices is not feasible insome designs. Thus, what are needed in the art are improved transistordesigns having low RTS noise.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a CMOS image sensor circuit in whichnovel transistors of embodiments of the present disclosure may beimplemented;

FIG. 2 is a top view of a layout of the circuit shown in FIG. 1;

FIG. 3 is a schematic of a circuit in accordance with an embodiment,with a top view of the transistor of the present disclosuresuper-imposed thereon;

FIGS. 4 and 5 show cross-sectional views of a method of manufacturing atransistor in accordance with an embodiment;

FIGS. 6 and 7 show cross-sectional views of a method of manufacturing atransistor in accordance with another embodiment;

FIGS. 8 and 9 show cross-sectional views of a method of manufacturing atransistor in accordance with yet another embodiment;

FIGS. 10 through 12 show cross-sectional views of a method ofmanufacturing a transistor in accordance with another embodiment;

FIG. 13 shows a cross-sectional view of a transistor in accordance withyet another embodiment of the present disclosure; and

FIG. 14 is a schematic diagram of another CMOS image sensor circuit inwhich the novel transistors of embodiments of the present disclosure maybe implemented, wherein a row select transistor is included in thecircuit.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the embodiments andare not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the embodiments of the present disclosure arediscussed in detail below. It should be appreciated, however, that thepresent disclosure provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the disclosure, and do not limit the scope of the disclosure.

Embodiments of the present disclosure are related to transistor designsfor CMOS image sensors. Novel transistor designs, methods of manufacturethereof, and image sensor circuits that utilize the novel transistorswill be described herein.

Referring first to FIG. 1, a schematic diagram 100 of a circuit in whicha novel transistor 108 may be implemented is shown, in accordance withan embodiment of the present disclosure. The schematic diagram 100 is acircuit of an image sensor that includes a photo diode 102 and a firsttransistor 104 coupled to the photo diode 102. A second transistor 106is coupled to the first transistor 104, and a third transistor 108 iscoupled to the first transistor 104 and the second transistor 106, asshown. A current source 110 is coupled to the third transistor 108. Avoltage threshold of the third transistor 108 is higher proximateshallow trench isolation (STI) regions 122 or 122′ (not shown in FIG. 1;see FIGS. 5, 9, 12, and 13) of the third transistor 108 in accordancewith embodiments of the disclosure, to be described further herein. Theincreased voltage threshold of the third transistor 108 proximate theSTI regions 122 or 122′ advantageously results in reduced RTS noise forthe third transistor 108 and the image sensor circuit 100 because morecurrent flows in a central region 150 of a channel 136 (see FIG. 13) ofthe third transistor 108, away from edge regions 152 which can be asource of RTS noise in source follower transistors.

The photo diode 102, the first transistor 104, the second transistor106, and the third transistor 108 comprise CMOS devices in accordancewith embodiments of the present disclosure. The third transistor 108 isalso referred to herein as a transistor or a source follower transistor.The first transistor 104, the second transistor 106, and the thirdtransistor 108 each comprise a source S, a drain D, and a gate G. Afirst side of the photo diode 102 is coupled to a return voltage orground, and a second side of the photo diode 102 is coupled to thesource S of the first transistor 104. The drain D of the firsttransistor 104 is coupled to the source S of the second transistor 106and the gate G of the third transistor 108, e.g., by wiring 112. Thesource S of the third transistor 108 is coupled to the current source110.

The third transistor 108 may comprise a source follower transistor andmay be coupled to a terminal Vsf for source follower voltage at thedrain D. An output voltage terminal Vo may be coupled to the source S ofthe third transistor 108. The first transistor 104 may comprise atransfer gate transistor and may be coupled to a terminal Vtx for atransfer gate voltage at the gate G. The second transistor 106 maycomprise a reset gate transistor and may be coupled to a terminal Vgrstfor a gate reset voltage at the gate G and coupled to a terminal Vrst atthe drain D, as shown. Alternatively, the circuit 100 may comprise otherconfigurations, for example.

The photo diode 102 of the image sensor circuit 100 is adapted toperform a photoelectric conversion, e.g., to accumulate a light signalcharge or retrieve information regarding an image the image sensorcircuit 100 is exposed to. The first transistor 104 comprising thetransfer gate transistor is adapted to transfer a signal charge obtainedat the photo diode 102 to the third transistor 108 comprising the sourcefollower transistor. The third transistor 108 is adapted to read out andchange an output voltage in accordance with an input voltage received ata predetermined voltage gain. The third transistor 108 may be adapted toamplify the signal transferred by the first transistor 104, for example.The second transistor 106 comprising the reset transistor is adapted todischarge a signal charge accumulated at the gate G of the secondtransistor 104 upon receipt of a reset signal received at voltageterminal Vgrst, for example.

The image sensor circuit 100 may comprise a read-out circuit for onepixel that includes a single photo diode 102 in an array of a pluralityof pixels (not shown) of a CMOS image sensor. The array may includehundreds or thousands of pixels arranged in a matrix, each comprising animage sensor circuit 100 shown in FIG. 1, as an example. The imagesensor circuit 100 may be implemented in a digital camera or otherimaging devices, for example.

FIG. 2 illustrates a top view of an exemplary layout of the circuit 100shown in FIG. 1; alternatively, the circuit 100 may comprise otherlayouts. FIG. 3 is a schematic of a circuit 100 in accordance with anembodiment with a top view of the transistor 108 of the presentdisclosure super-imposed thereon, illustrating an arrangement andconnections of the source S, drain D, and gate G of the transistor 108over a workpiece 120 (not shown in FIG. 3; see FIG. 4). Cross-sectionalviews of the transistor 108 at A-A′ above and below the gate 114 areshown in FIGS. 4 through 13, to be described further herein. Fourembodiments of methods of forming transistors 108 having a high voltagethreshold proximate STI regions 122 and 122′ will be described herein,with reference to FIGS. 4 through 13. These embodiments illustratealternative approaches to forming a voltage threshold modificationfeature, which feature will cause the voltage threshold of edge regionsof the channel to be higher relative to a central region of the channel.

FIGS. 4 and 5 show cross-sectional views of a method of manufacturing atransistor 108 in accordance with a first embodiment, wherein STIregions 122′ are implanted with a dopant on the sidewalls 128 and bottomsurfaces 130 (see FIG. 4), forming an implantation region 126, beforebeing filled with an insulating material 138 (see FIG. 5). Thetransistor 108 comprises a gate dielectric 134 disposed over a workpiece120 proximate a shallow trench isolation (STI) region 122′ and a gate114 disposed over the gate dielectric 134.

To manufacture the transistor 108, a workpiece 120 is first provided,shown in FIG. 4. The workpiece 120 may include a semiconductor substratecomprising silicon or other semiconductor materials and may be coveredby an insulating layer, for example. The workpiece 120 may also includeother active components or circuits, not shown. The workpiece 120 maycomprise silicon oxide over single-crystal silicon, for example. Theworkpiece 120 may include other conductive layers or other semiconductorelements, e.g., transistors, diodes, etc. Compound semiconductors, GaAs,InP, Si/Ge, or SiC, as examples, may be used in place of silicon. Theworkpiece 120 may comprise a silicon-on-insulator (SOI) or agermanium-on-insulator (GOI) substrate, as examples. The workpiece 120may comprise one or more layers of electrical circuitry and/orelectronic functions formed thereon, and may include conductive lines,vias, capacitors, diodes, transistors, resistors, inductors, and/orother electrical components formed in a front end of the line (FEOL)process, for example (not shown).

STI regions 122′, also referred to herein as isolation regions, areformed in the workpiece 120 by forming a mask 132 over the workpiece120, and patterning the mask 132 with a desired pattern for the STIregions 122′ using lithography. The mask 132 may comprise siliconnitride or other insulating materials, for example. The mask 132 is thenused as a mask during an etch process or other removal process to removetop portions of the exposed workpiece 120 and form trenches 124 for theSTI region 122′, as shown. The trenches 124 extend in and out of thepaper lengthwise, for example. The trenches 124 comprise sidewalls 128and a bottom surface 130. Only one trench 124 is shown in FIG. 4;however, two trenches 124 may be formed for each transistor 108 or acontinuous trench may be formed encircling a region of workpiece 120wherein transistor 108 is to be formed, as shown in FIG. 5. A pluralityof trenches 124 may be formed across a surface of the workpiece 120 foran array of image sensor circuits 100, for example, not shown.

In accordance with the first embodiment, the sidewalls 128 and thebottom surface 130 of the trenches 124 are implanted with a dopant,forming an implantation region 126. The mask 132 is left remaining onthe surface of the workpiece 120 during the implantation process toprotect the top surface of the workpiece 120 from being implanted. Theimplantation process of the trenches 124 may comprise implanting B, P,or other substances at an angle of about 60 degrees or less, and in someembodiments at an angle of about 10 to 45 degrees, at a dosage of about1×10¹¹ to 1×10¹³/cm², as an example, although alternatively, otherimplantation processes and parameters may be used. If the thirdtransistor 108 comprises an NMOS transistor, then B can be implanted toform an implantation region 126 comprising p-type doping; oralternatively, if the third transistor 108 comprises a PMOS transistor,then P can be implanted to form an implantation region 126 comprisingn-type doping, as examples. The implantation region 126 may comprisesubstantially the same dopant concentration or a different dopantconcentration on the bottom surface 130 and sidewalls 128.

The trenches 124 are filled with an insulating material 138, as shown inFIG. 5. The insulating material 138 may comprise silicon dioxide,silicon nitride, other insulating materials, or multiple layers orcombinations thereof, as examples. Excess insulating material 138 andthe mask 132 are removed from the top surface of the workpiece 120,using one or more chemical mechanical polishing and/or etch processes,exposing the top surface of the workpiece 120 and the insulatingmaterial 138. The STI regions 122′ comprise the implantation regions 126and the insulating material 138. The STI regions 122′ comprise an edgehigher doping concentration due to the implantation regions 126.

A gate dielectric material 134 comprising an insulating material isformed over the workpiece 120 and STI regions 122′, and a gate material114 comprising a conductive material, a semiconductive material, ormultiple layers or combinations thereof is formed over the gatedielectric material 134. The gate material 114 and the gate dielectricmaterial 134 are patterned using lithography to form a gate 114 and gatedielectric 134 of the transistor 108 disposed over the workpiece 120.The gate 114 and gate dielectric 134 extend (e.g., in and out of thepaper) lengthwise along sides of the STI regions 122′ in a top view. Aportion of the gate 114 and gate dielectric 134 may reside over the STIregions 122′ on either side, as shown. Alternatively, a portion of thegate 114 and gate dielectric 134 may not reside over a portion of theSTI regions 122′, for example. A channel 136 is formed beneath the gatedielectric 134 within the workpiece 120, as shown. Note that the sourceand drain regions S and D of the transistor 108 are into and out of theplane of the page, and the channel width 136 is illustrated from left toright in the drawings.

The implantation regions 126 increase the voltage threshold of thetransistor 108 in edge regions 152 compared to center regions 150 of thechannel 136 proximate the STI regions 122′, preventing or reducingcurrent flow at edge regions 152 proximate the STI regions 122′ duringthe operation of the transistor 108 and reducing RTS noise,advantageously. The higher doping concentration of the implantationregions 126 at the STI region 122′ edges, e.g., at the well of the STIregion 122′, causes current to tend not to flow at the STI region 122′edge, for example.

FIGS. 6 and 7 show cross-sectional views of a method of manufacturing atransistor 108 in accordance with a second embodiment. Like numerals areused for the various elements in FIGS. 6 and 7 (and also in FIGS. 8through 13) that were used to describe FIGS. 1 through 5. To avoidrepetition, each reference number shown in FIGS. 6 and 7 is notdescribed again in detail herein. Rather, similar materials 100, 102,104, 106 etc., are used to describe the various material layers andcomponents shown as were used to describe the previous FIGS. 1 through5.

In this embodiment, edge regions 135 of the gate dielectric 134′ alongthe length of the gate 114 are increased in thickness to achieve ahigher voltage threshold proximate the STI regions 122 than in centralregions 133 of the gate dielectric 134′ proximate the central regions150 of the channel 136.

To increase the thickness of the gate dielectric 134, the gatedielectric material 134 is formed over the workpiece 120. In theembodiment shown, the STI region 122 trenches 124 have not yet beenfilled with an insulating material 138; alternatively, the trenches 124may be filled with an insulating material 138 (see FIG. 5) prior toforming the gate dielectric material 134, for example, not shown. Thegate dielectric material 134 may be deposited or grown over theworkpiece 120, for example. The gate dielectric material 134 has athickness comprising dimension d₁ as initially formed, wherein dimensiond₁ comprises about 20 to 100 Angstroms, for example. Alternatively,dimension d₁ may comprise other values.

A mask 132 is formed over the gate dielectric material 134, as shown inFIG. 6. The mask 132 may comprise silicon nitride or other materials,for example. The mask 132 is patterned and used as a mask during an etchprocess which forms the gate dielectric 134 and optionally also formsthe trenches 124 for the STI regions 122 in this embodiment, forexample. The mask 132 is exposed to an etch-back process or pull-backprocess, removing a portion of the mask 132 from the top surface of edgeregions 135 of the gate dielectric 134 and optionally from other regionsof the workpiece 120, as shown in FIG. 6.

The workpiece 120 is then exposed to an implantation process 140, asshown in FIG. 7, which comprises implanting a substance into the gatedielectric 134 in the edge regions 135. The substance implantedcomprises Ar in some embodiments, for example. In these embodiments, athermal oxidation process may then be used to increase the thickness ofthe gate dielectric 134 in the edge regions 135. The substance implantedmay also comprise an oxide, as another example, which increases thethickness of the gate dielectric 134′ in the edge regions 135.

The thickness of the gate dielectric 134′ in the edge regions 135 maycomprise a dimension d₂, wherein dimension d₂ comprises about 30 to 120Angstroms, for example. Alternatively, dimension d₂ may comprise othervalues. In some embodiments, dimension d₂ is about 10 to 30% greaterthan dimension d₁, for example.

FIGS. 8 and 9 show cross-sectional views of a method of manufacturing atransistor 108 in accordance with yet another embodiment. The thicknessof the gate dielectric 134′ proximate the STI regions 122 is increasedin this embodiment by forming a masking material 144 over the workpiece120, before forming the gate dielectric material 134′, as shown in FIG.8. The masking material 144 is patterned, exposing portions of theworkpiece 120 proximate the STI regions 122. Note that the STI regions122 comprise only an insulating material 138 and not the implantationregions 126 shown in the first embodiment. A substance is then implantedinto the workpiece 120 proximate the STI regions 122 using animplantation process 142, and the masking material 144 is removed. Theimplantation process 142 may comprise implanting Ar in some embodiments,for example. The implantation process 142 results in the formation ofimplantation regions 146 in regions of the workpiece 120 proximate theSTI regions 122, as shown.

The gate dielectric material 134′ is then deposited or formed over theworkpiece 120, as shown in FIG. 9. The gate dielectric material 134′ maybe formed using an oxidation process or other methods, for example. Thepresence of the implanted substance in implantation regions 146 resultsin a thicker gate dielectric 134′ in edge regions 135 being formed, dueto the presence of the underlying implanted substance in implantationregions 146, as shown.

The gate dielectric 134′ may be formed using a thermal oxidationprocess, for example, and the presence of the implanted Ar causes anincreased thickness of gate dielectric 134′ over the implantationregions 146 at the edge regions 135 compared to non-implanted regions ofthe workpiece 120 where the gate dielectric 134′ formed is thinner.Implanted argon ions in a substrate enhance oxide growth, for example.

The increased thickness of the gate dielectric 134′ proximate the edgesof the isolation regions 122 results in an increase in the voltagethreshold of the transistor 108 in the second and third embodimentsshown in FIGS. 6, 7, 8 and 9 proximate the isolation regions 122. Thegate dielectric 134′ edge regions 135 proximate the STI regions 122 maycomprise a thickness of dimension d₂ that is about 10 to 30% greaterthan a thickness comprising dimension d₁ of the gate dielectric 134′ inthe central region 133, e.g., disposed over the central region 150 ofthe channel 136 of the transistor 108, in some embodiments. The edgeregions 135 may comprise a distance apart from the STI regions 122 onsides of the gate dielectric 134′ comprising dimensions d₃ and d₄,wherein dimensions d₃ and d₄ may be about 0.03 μm to 0.1 μm, forexample, although alternatively, dimension d₃ may comprise other values.Dimensions d₃ and d₄ may comprise substantially the same value or maycomprise different values in other embodiments, for example. The thickergate dielectric 134′ edge regions 135 along the gate 114 lengthincreases the voltage threshold at the STI region 122 edge so thatcurrent tends not to flow at the STI region 122 edge. The higher voltagethreshold of the transistor 108 proximate the isolation regions 122results in reduced RTS noise.

FIGS. 10 through 12 show cross-sectional views of a method ofmanufacturing a transistor 108 in accordance with a fourth embodiment.The gate 114′ comprises a greater amount of implanted dopants in acentral region 154 of the gate 114′ than in edge portions 156 of thegate 114′ proximate the STI regions 122 in this embodiment. After theformation of STI regions 122 comprising insulating material 138 in theworkpiece 120, a gate dielectric material 134 is formed over theworkpiece 120 and STI regions 122, as shown in FIG. 10. A gate material114 that may comprise a semiconductive material such as polysilicon orother materials is formed over the gate dielectric material 134. Thegate material 114 may be undoped or lightly doped, for example. The gatematerial 114 may be lightly doped with N+ material at a concentration ofabout 1×10¹¹ to 1×10¹²/cm², as an example, although other dopingconcentrations and materials may also be used. A mask 144 that maycomprise a photoresist or a combination of a photoresist and a hardmask, as examples, is deposited over the gate material 114, as shown.The mask 144 is patterned using lithography to open a region in the mask144 over the gate material 114.

The workpiece 120 is exposed to an implantation process 147 to form ahigher doping concentration in a central region 154 of the gate 114′than in edge regions 156, as shown in FIG. 11. The implantation process127 may comprise implanting B, P, or As at about 10 Key to 200 Key at adosage of about 1×10¹⁴ to 1×10¹⁵/cm², as examples, althoughalternatively, other implantation processes and parameters may be used.The edge regions 156 may comprise a distance apart from the STI regions122 on sides of the channel 136 comprising dimensions d₅ and d₆, whereindimensions d₅ and d₆ may be about 0.03 μm to 0.1 μm, for example,although alternatively, dimensions d₅ and d₆ may comprise other values.Dimensions d₅ and d₆ may comprise substantially the same value or maycomprise different values in other embodiments, for example. The gatematerial 114′ and the gate dielectric 134 are then patterned, leavingthe transistor 108 shown in FIG. 12 including gate 114′ that has beenaltered to have a lower doping concentration at edge regions 156 than atthe central region 154. The lower doping concentration at edge regions156 of the gate 114′ along the gate 114′ length increases the voltagethreshold of the transistor 108 proximate edges of the STI region 122and proximate the edge regions 152 of the channel 136 so that currenttends not to flow through the STI region 122 edges, which reduces oreliminates RTS noise, advantageously.

FIG. 13 shows a cross-sectional view of a transistor 108 in accordancewith yet another embodiment. Combinations of the four embodimentspreviously described may be implemented in a transistor 108 inaccordance with the present disclosure. In the embodiment shown, thefirst embodiment shown in FIGS. 4 and 5, and the fourth embodiment shownin FIGS. 10 through 12 have been implemented to achieve an STI region122′ comprising implantation region 126 and insulating material 138, anda gate 114′ having a central region 154 with a high dopingconcentration, respectively. Either the second embodiment shown in FIGS.6 and 7, or the third embodiment shown in FIGS. 8 and 9 has also beenimplemented to achieve a gate dielectric 134′ that is thicker at theedge regions 135 than at the center regions 133. Likewise, any two ormore of the embodiments may be implemented in the manufacturing processof a transistor 108 in other combinations.

Portions of the workpiece 120 may be masked during the variousmanufacturing processes for the third transistors 108 described hereinin some embodiments. In other embodiments, other portions of the CMOSimage sensor circuit 100 may be simultaneously formed during themanufacturing of the third transistors 108, such as portions of thephoto diode 102, the first transistor 104, the second transistor 106,wiring 112, and other elements of the circuit 100, not shown.

Advantages of embodiments of the disclosure include providing noveltransistor 108 designs that have reduced RTS noise and that do notrequire that their size (e.g., the gate 114/114′ size in a top view) beincreased to reduce the RTS noise. Embodiments have been describedherein wherein an STI region 122′, a gate dielectric 134′, and/or a gate114′ are modified or altered to achieve a higher voltage thresholdproximate edge regions 152 of a channel 136, proximate STI regions122/122′, than at a central region 150 of the channel 136, so that agreater amount of current is passed through the central region 150 ofthe channel 136 during operation of the transistor 108, which reducesRTS noise by avoiding or reducing current flow in edge regions 152 ofthe channel 136, advantageously. The higher voltage thresholds at theedges of the STI regions 122/122′ cause current flow through the sourcefollower transistor 108 gate to avoid the STI region 122/122′ edge.

The novel transistors 108 are easily implementable in manufacturingprocess flows for image sensor circuits 100. The transistors 108described herein having a higher voltage threshold at the STI region122/122′ edges are particularly useful in reducing or eliminating RTSnoise when used as source follower transistors of CMOS image sensors oractive pixel sensors (APSs), for example. A voltage applied to thetransistors 108 creates a greater amount of current through the centralregion 150 of the channel 136 than in the edge regions 152 of thechannel 136.

Embodiments of the present disclosure also have useful application inother types of APSs. As an example, embodiments may be implemented in anAPS having a row select transistor 160 coupled thereto, as shown in FIG.14, which is a schematic diagram 100 of another CMOS image sensorcircuit in which the novel transistors of embodiments of the presentdisclosure may be implemented, wherein a source follower transistor 108described herein is included in the APS circuit. The row selecttransistor 160 comprises a fourth transistor of the circuit that iscoupled between the source follower transistor 108 and the currentsource 110, as shown. The source S of the source follower transistor 108is coupled to a drain D of the row select transistor 160, and a source Sof the row select transistor 160 is coupled to the current source 110.An output voltage terminal Vo may be coupled to the source S of the rowselect transistor 160. A voltage V may be applied to a gate G of the rowselect transistor 160 to select a row of pixels in an array of pixels,for example.

Embodiments of the present disclosure include the novel transistor 108designs and methods of manufacturing the transistors 108 describedherein. Embodiments of the present disclosure also include image sensorcircuits 100 that include the transistors 108 described herein.

In accordance with one embodiment of the present disclosure, atransistor includes a channel disposed between two isolation regions ina workpiece. The channel has edge regions proximate the isolationregions and a central region between the edge regions. The transistorincludes a gate dielectric disposed over the channel, and a gatedisposed over the gate dielectric. The transistor includes a voltagethreshold modification feature proximate the edge regions configured toincrease a voltage threshold of the transistor proximate edge regionsrelative to the central region of the channel.

In accordance with another embodiment, a method of manufacturing atransistor includes providing a workpiece, patterning the workpiece toform two isolation regions in the workpiece, and forming a gatedielectric over the workpiece between the isolation regions. A gate isformed over the gate dielectric, and a voltage threshold modificationfeature is formed proximate the isolation regions.

In accordance with yet another embodiment, an image sensor circuitincludes a photo diode, a first transistor coupled to the photo diode,and a second transistor coupled to the first transistor. A thirdtransistor is coupled to the first transistor and the second transistor.The third transistor includes a channel disposed between two isolationregions in a workpiece, a gate dielectric disposed over the channel, anda gate disposed over the gate dielectric. A voltage threshold of thethird transistor is higher proximate the isolation regions than in acentral region of the channel.

Transistors, methods of manufacturing thereof, and image sensor circuitswith reduced random telegraph signal (RTS) noise are disclosed. In oneembodiment, a transistor includes a channel disposed between twoisolation regions in a workpiece. The channel has edge regions proximatethe isolation regions and a central region between the edge regions. Thetransistor includes a gate dielectric disposed over the channel, and agate disposed over the gate dielectric. The transistor includes avoltage threshold modification feature proximate the edge regionsconfigured to increase a voltage threshold of the transistor proximateedge regions relative to the central region of the channel.

In accordance with yet another embodiment, a method of forming asemiconductor device is provided. The method includes forming a firsttrench and a second trench in a substrate, and implanting dopants of afirst conductivity type along sidewalls and a bottom of the first trenchand the second trench. The first trench and the second trench is filledwith an insulating material. A gate dielectric and a gate electrode isformed over the substrate, wherein the gate dielectric and the gateelectrode extend over the first trench and the second trench.Source/drain regions are formed in the substrate on opposing sides ofthe gate dielectric and the gate electrode.

In accordance with yet still another embodiment, a method of forming asemiconductor device is provided. The method includes forming a firsttrench and a second trench in a substrate, and forming a gate dielectricand a gate electrode over the substrate between the first trench and thesecond trench. Source/drain regions are formed in the substrate onopposing sides of the gate dielectric and the gate electrode, whereinthe gate electrode has a gate length extending between the source/drainregions, and a gate width perpendicular to the gate length. The gateelectrode includes a first electrode peripheral region, a secondelectrode peripheral region, and a central electrode region interposedbetween the first electrode peripheral region and the second peripheralregion along a direction of the gate width, wherein the gate electrodeis doped such that a dopant concentration in the first electrodeperipheral region and the second electrode peripheral region is lessthan a dopant concentration in the central electrode region.

In yet still another embodiment, a method of forming a semiconductordevice is provided. The method includes forming a first trench and asecond trench in a substrate, and forming a gate dielectric between thefirst trench and the second trench. The gate dielectric has a firstdielectric peripheral region closest to the first trench, a seconddielectric peripheral region closest to the second trench, and a centraldielectric region interposed between the first dielectric peripheralregion and the second dielectric peripheral region, wherein the firstdielectric peripheral region and the second dielectric peripheral regionextend over an uppermost surface of the substrate immediately adjacentthe first trench and the second trench, respectively. The gatedielectric has a first thickness in the first peripheral region, asecond thickness in the second dielectric peripheral region, and a thirdthickness in the central dielectric region, wherein the first thicknessand the second thickness is greater than the first thickness. A gateelectrode is formed over the gate dielectric, and source/drain regionsare formed in the substrate on opposing sides of the gate dielectric andthe gate electrode. The gate electrode has a gate length that extendsbetween the source/drain regions, and a gate width perpendicular to thegate length, wherein the gate width extends in a direction from thefirst trench to the second trench.

In yet still another embodiment, an image sensor is provided. The imagesensor includes a photo diode, a first transistor coupled to the photodiode, a second transistor coupled to the first transistor, and a thirdtransistor coupled to the first transistor and the second transistor.The third transistor includes a channel disposed between two isolationregions in a semiconductor material, a gate dielectric disposed over thechannel, and a gate disposed over the gate dielectric, wherein a voltagethreshold of the third transistor is higher proximate the isolationregions than in a central region of the channel.

In yet still another embodiment, an image sensor is provided. The imagesensor includes a photo sensor, a first transistor electrically coupledto the photo sensor, a second transistor electrically coupled to thefirst transistor, and a third transistor electrically coupled to thefirst transistor and the second transistor. The third transistorincludes a channel disposed between two isolation regions in aworkpiece, a gate dielectric disposed over the channel, and a gatedisposed over the gate dielectric, the gate having a first portionadjacent the first isolation region, a second portion adjacent thesecond isolation region and a third portion interposed between the firstportion and the second portion, the third portion of the gate having ahigher dopant concentration than the first portion and the secondportion of the gate.

In yet still another embodiment, an image sensor is provided. The imagesensor includes a photo diode, a first transistor electrically coupledto the photo diode, a second transistor electrically coupled to thefirst transistor, and a third transistor electrically coupled to thefirst transistor and the second transistor. The third transistorincludes a channel disposed between two isolation regions in aworkpiece, a gate dielectric disposed over the channel, and a gatedisposed over the gate dielectric, the gate dielectric having a firstportion adjacent the first isolation region, a second portion adjacentthe second isolation region and a third portion interposed between thefirst portion and the second portion of the gate dielectric, the firstportion and the second portion of the gate dielectric having a greaterthickness than the third portion of the gate dielectric.

Although embodiments of the present disclosure and their advantages havebeen described in detail, it should be understood that various changes,substitutions and alterations can be made herein without departing fromthe spirit and scope of the disclosure as defined by the appendedclaims. For example, it will be readily understood by those skilled inthe art that many of the features, functions, processes, and materialsdescribed herein may be varied while remaining within the scope of thepresent disclosure. Moreover, the scope of the present application isnot intended to be limited to the particular embodiments of the process,machine, manufacture, composition of matter, means, methods and stepsdescribed in the specification. As one of ordinary skill in the art willreadily appreciate from the disclosure of the present disclosure,processes, machines, manufacture, compositions of matter, means,methods, or steps, presently existing or later to be developed, thatperform substantially the same function or achieve substantially thesame result as the corresponding embodiments described herein may beutilized according to the present disclosure. Accordingly, the appendedclaims are intended to include within their scope such processes,machines, manufacture, compositions of matter, means, methods, or steps.

What is claimed is:
 1. An image sensor, comprising: a photo diode; afirst transistor electrically coupled to the photo diode; a secondtransistor electrically coupled to the first transistor; and a thirdtransistor electrically coupled to the first transistor and the secondtransistor, wherein the third transistor comprises a channel disposedbetween two isolation regions in a semiconductor material, a gatedielectric disposed over the channel, and a gate disposed over the gatedielectric, wherein a voltage threshold of the third transistor ishigher proximate the isolation regions than in a central region of thechannel, wherein the gate has a higher dopant concentration over thecentral region of the channel than in outer regions adjacent respectiveones of the two isolation regions, wherein the central region isinterposed between the outer regions.
 2. The image sensor of claim 1,wherein the first transistor comprises a transfer gate transistor, thesecond transistor comprises a reset gate transistor, and the thirdtransistor comprises a source follower transistor.
 3. The image sensorof claim 1, wherein the first transistor comprises a source, a drain,and a gate, wherein the second transistor comprises a source, a drain,and a gate, wherein the third transistor comprises a source, a drain,and a gate, wherein the photo diode comprises a first side and a secondside, and wherein a first side of the photo diode is electricallycoupled to a return voltage, a second side of the photo diode iselectrically coupled to the source of the first transistor, and thedrain of the first transistor is electrically coupled to the source ofthe second transistor and the gate of the third transistor.
 4. The imagesensor of claim 3, further comprising a fourth transistor having asource, a drain, and a gate, wherein the drain of the fourth transistoris electrically coupled to the source of the third transistor, whereinthe source of the fourth transistor is electrically coupled to a currentsource, and wherein the fourth transistor comprises a row selecttransistor.
 5. The image sensor of claim 1, wherein the gate dielectrichas a greater thickness over an uppermost surface of the semiconductormaterial immediately adjacent the isolation regions than over a centralpart of the channel.
 6. An image sensor, comprising: a photo sensor; afirst transistor electrically coupled to the photo sensor; a secondtransistor electrically coupled to the first transistor; and a thirdtransistor electrically coupled to the first transistor and the secondtransistor, wherein the third transistor comprises a channel disposedbetween a first isolation region and a second isolation region in aworkpiece, a gate dielectric disposed over the channel, and a gatedisposed over the gate dielectric, the gate having a first portionadjacent the first isolation region, a second portion adjacent thesecond isolation region and a third portion interposed between the firstportion and the second portion, the third portion of the gate having ahigher dopant concentration than the first portion and the secondportion of the gate.
 7. The image sensor of claim 6, wherein each of thefirst isolation region and the second isolation region comprises atrench in a semiconductor material, further comprising an implant regionalong sidewalls and a bottom surface of each trench.
 8. The image sensorof claim 7, wherein the implant region comprises an N-type or P-typedopant.
 9. The image sensor of claim 6, wherein the gate dielectric hasa first portion, a second portion, and a third portion interposedbetween the first portion and the second portion of the gate dielectric,the first portion and the second portion of the gate dielectric having agreater thickness than the third portion of the gate dielectric.
 10. Theimage sensor of claim 9, wherein the first portion and the secondportion of the gate dielectric comprises Ar dopants.
 11. The imagesensor of claim 10, wherein the gate does not extend over the firstportion and the second portion of the gate dielectric.
 12. An imagesensor, comprising: a photo diode; a first transistor electricallycoupled to the photo diode; a second transistor electrically coupled tothe first transistor; and a third transistor electrically coupled to thefirst transistor and the second transistor, wherein the third transistorcomprises a channel disposed between a first isolation region and asecond isolation region in a workpiece, a gate dielectric disposed overthe channel, and a gate disposed over the gate dielectric, the gatedielectric having a first portion adjacent the first isolation region, asecond portion adjacent the second isolation region and a third portioninterposed between the first portion and the second portion of the gatedielectric, the first portion and the second portion of the gatedielectric having a greater thickness than the third portion of the gatedielectric, wherein the first portion and the second portion of the gatedielectric are free of the gate.
 13. The image sensor of claim 12,wherein the gate is disposed above the third portion of the gatedielectric.
 14. The image sensor of claim 13, wherein the gatedielectric does not extend over the first isolation region and thesecond isolation region.
 15. The image sensor of claim 12, wherein adopant concentration in the gate is non-uniform.
 16. The image sensor ofclaim 15, wherein a first region of the gate has a first dopantconcentration, a second region of the gate has a second dopantconcentration, and a third region of the gate has a third dopantconcentration, wherein the first region is interposed between the secondregion and the third region, and wherein the first dopant concentrationis less than the second dopant concentration and the third dopantconcentration.
 17. The image sensor of claim 12, wherein the firstisolation region and the second isolation region each comprise a trenchin a semiconductor material, and further comprising implantation regionsin the semiconductor material of the workpiece along sidewalls and abottom surface of the trench.
 18. The image sensor of claim 12, whereina thickness of the first portion of the gate dielectric is 10% to 30%greater than a thickness of the third portion of the gate dielectric.19. The image sensor of claim 1, wherein the outer regions overlaprespective ones of the isolation regions.
 20. The image sensor of claim1, wherein a region of the gate having the higher dopant concentrationis spaced apart from an edge of one of the isolation regions by adistance from 0.03 μm to 0.1 μm.